1. Technical Field
The present invention relates to an apparatus for detecting knocking occurring in an internal combustion.
2. Related Art
For driving an internal combustion, it is usually required to have an apparatus for detecting knocking occurring in the internal combustion. Conventional techniques for detecting the occurrence of knocking have already been proposed by various references.
One example is disclosed by a Japanese Patent Laid-open (unexamined) publication No. 7-293314 (refer to pages 2-3 and FIG. 5 thereof).
This reference discloses determination as to whether or not there occurs a knocking of an internal combustion. Practically, a knocking sensor is used to detect a signal indicative of a knocking of an internal combustion. This signal from the knocking sensor is subjected to A/D conversion at specified sampling intervals. The A/D-converted data, which are outputted sequentially in time, is sent to a plurality of digital filters whose pass ranges are different from each other. Outputs from those digital filters are used to find out whether or not the internal combustion has caused a knocking phenomenon in an engine. In the example shown by the above reference, the plural digital filters include two digital filters: one passes a signal whose frequencies have a peak in response to the occurrence of a knocking (that is, a knocking signal), and the other passes a signal whose frequencies have a peak in response to the occurrence of noise (that is, a noise signal other than the knocking signal).
Another reference is a Japanese Patent Laid-open (unexamined) 2001-164982, which teaches an idea of using a filter filtering a knocking signal from an oscillation signal wave from a knocking sensor. This reference teaches that it is desirable that the bandwidth (Q value) of the filter be a value not so higher, for example, less than 10 dB. Further, there is another reference, a Japanese Patent Laid-open (unexamined) No. 5-26721 (refer to pages 2 and 5), which teaches that the frequency of knocking oscillation varies depending on operation states of an engine. Another reference is a Japanese Patent Laid-open (unexamined) 7-109949, which discloses the teaching that a knocking sensor signal including signal components indicative of knocking and noise components and the spectrum of the signal components is larger in amplitude than the spectrum of the noise components. On the other hand, it has been known that, as the digital filter, an FIR (finite impulse response) filter and an IIR (infinite impulse response) filter can be used. As the IIR filter, there are filters of such characteristics as Butterworth characteristics and the Chebyshev characteristics.
However, in the case that, as suggested by the foregoing reference (publication No. 7-293314), the outputs from the plural filters are used to determine whether or not there occurs a knocking phenomenon in a combustion engine, there still exist some problems as follows.
The problems will now be explained with reference to FIGS. 13 and 14, each of which exemplifies the filtering characteristics of five filters filt1 to filt5 for knocking monitor (in the figures, the vertical and horizontal axes denote a gain and a frequency). In both the figures, the filtering characteristic of each filter filt1 (to filt5) is expressed by a dashed line, while a knocking sensor signal is expressed by a solid line. The knocking sensor signal is shown in the form of a signal modified, with the horizontal-axis values being logarithmic, from that shown in FIG. 16 of the foregoing reference (publication No. 7-109949), in which the signal is obtained when the knocking occurs. As to the knocking sensor signal, the vertical axis in FIGS. 13 and 14 indicates the spectrum amplitude. Each of the filters filt1 to filt5 shown in FIGS. 13 and 14 is composed of a forth-order band-pass filter formed by combining a second-order low-pass filter and a second-order high-pass filter with each other.
Of the five filters filt1 to filt5, each of three filters filt1 to filt3 has a pass band assigned to a specific frequency range including each of specific frequencies of the knocking sensor signal. The specific frequencies, which clearly indicate the occurrence of knocking, are, for example, 7 kHz, 12 kHz (corresponding to the 1.5-th order harmonic of 7 kHz), and 15.5 kHz (corresponding to the second harmonic of 7 kHz). That is, those three filters filt1 to filt3 are set to have three passing frequency bands of predetermined bandwidths each having center frequencies each of 7 kHz, 12 kHz, and 15.5 kHz serving as three knocking frequencies. Hereinafter, the filters filt1 to filt3 are referred to as “knocking-signal filters.”
The remaining two filters filt4 and filt5 have passing frequency bands, which are different from the foregoing frequency bands for the three knocking frequencies and assigned to specific frequency ranges including each of specific frequencies which can be regarded as noise within the knocking sensor signal. In this example, such specific frequencies are 3 kHz and 4.5 kHz corresponding to the 1.5-th order harmonic of 3 kHz. That is, those two filters filt4 to filt5 are set to have two passing frequency bands of predetermined bandwidths each having center frequencies each of 3 kHz and 4.5 kHz. Hereinafter, the filters filt4 and 5 are referred to as “noise filters.”
As understood from FIG. 13, when the Q values of the filters filt1 to filt5 are low (in this example, Q=10 dB), there is a fear that the noise filter filt5 is influenced by the knocking signal. Specifically, the filter filt5 whose center frequency f0=4.5 kHz still has a considerably higher attenuation rate at a knocking frequency of 7 kHz, while the knocking signal is larger in amplitude than the noise. Hence the presence of the knocking signal 7 kHz affects the noise filter filt5.
Accordingly, as shown in FIG. 13, in cases where all of the knocking signal filters filt1 to filt3 and the noise filters filt4 and filt5 are given low Q values, one or more noise filters frequently result in response to knocking occurring at an internal combustion. This deteriorates an S/N ratio, whereby the knocking cannot be detected with precision.
In contrast, FIG. 14 exemplifies a case in which the knocking signal filters filt1 to filt3 and the noise filters filt4 and filt5 are given low Q values (in this example, Q=25 dB). In this case, it cannot be expected to have the knocking detected with precision, if the frequencies of the knocking signal (i.e., the knocking frequencies) are shifted due to operation conditions and/or ageing of an engine. Namely, even when the knocking frequencies are shifted slightly, those frequencies are forced to be filtered at largely different gains (especially, to the filters filt1 to filt3).